The present invention relates to rolls for the treatment of running webs, especially to improvements in rolls for pressure treatment of running paper webs in paper making and/or processing machines.
U.S. Pat. No. 3,846,883 granted Nov. 12, 1974 to Biondetti discloses a roller for pressure treatment of webs. The roller comprises a flexible shaft and a rigid sleeve which spacedly surrounds the shaft and rotates in response to engagement with a running web of paper or the like. The means for transmitting forces between the sleeve and the shaft comprises several bearing shoes which have convex cylindrical surfaces adjacent to the concave cylindrical internal surface of the sleeve. In the embodiment which is illustrated in FIG. 5 of the patent to Biondetti, the convex cylindrical surface of each shoe has several hydrostatic bearing pockets which are filled with pressurized hydraulic fluid to prevent metal-to-metal contact between the sleeve and the shoes. Rolls of the just outlined character can be used in paper making machines to expel surplus moisture from freshly formed paper webs or in calenders to enhance the smoothness of or to otherwise treat the surfaces of running webs consisting of paper or the like. Furthermore, such rolls can be used in many other types of machines wherein discrete sheets or continuous webs or strips of flexible material are caused to pass through the nips of rolls which are driven to advance the webs or sheets or which receive motion in response to entrainment of sheets or webs therebetween.
The purpose of bearing shoes between the shaft and the rotatable sleeve or shell is that the sleeve is much less likely to undergo deformation so that the width of the nip remains constant from one end and all the way to the other end of the roll. Furthermore, the bearing shoes render it possible to vary the pressure against the internal surface of the sleeve from section to section, as considered in the axial direction of the roll, so as to ensure that the resistance to deformation is greatest in the region where the sleeve is most likely to be deformed. This also contributes to the possibility of ensuring that the width of the nip remains constant all the way between the ends of the cooperating rolls.
The external cylindrical surface of each bearing shoe is complementary to the internal cylindrical surface of the shell. In the embodiment which is shown in FIG. 5 of the patent to Biondetti, the external surface of each bearing shoe has a centrally located hydrostatic bearing pocket and two or more additional bearing pockets in circumferential orientation around the central pocket. The patentee suggests that this substantially reduces friction between the stabilizing edge around the central pocket and the shell. The two additional pockets which are actually shown in FIG. 5 of Biondetti are disposed at the opposite sides of the central pocket, as considered in the circumferential direction of the shell.
FIG. 3 of U.S. Pat. No. 3,802,044 granted Apr. 9, 1974 to Spillmann et al. discloses a modified bearing shoe with a circular external surface which is formed with four equal sector-shaped hydrostatic bearing pockets. Two of the pockets are halved by a plane which includes the longitudinal axis of the sleeve, and the remaining two pockets are halved by a plane which is normal to the longitudinal axis of the sleeve. The webs or lands which separate the neighboring pockets from each other make angles of approximately 45 degrees with the longitudinal axis of the sleeve as well as with a transverse axis which is normal to and intersects the longitudinal axis. It can be said that the lands or webs between the neighboring sector-shaped pockets extend diagonally of the exposed cylindrical surface of the bearing shoe.
The hydrostatic pockets which are disclosed by Biondetti and Spillmann et al. can stabilize the respective bearing shoes. Such shoes must be capable of performing various movements relative to the shaft. As a rule, the bearing shoes are mounted for movement radially of the shaft as well as for tilting movement not unlike the parts of universal joints. The stabilizing effect of the hydrostatic pockets is to be felt in several directions, namely, as considered in the axial direction as well as in the circumferential direction of the sleeve (i.e., in the direction of as well as at right angles to the central longitudinal axis of the sleeve). Otherwise stated, the hydrostatic bearing pockets of the aforediscussed conventional bearing shoes ensure that the axis of each bearing shoe extends exactly radially of the associated sleeve or shell. The patent to Spillmann et al. further discloses the possibility of rotating the array of pockets shown in FIG. 3 through 45 degrees so that two of the lands between neighboring bearing pockets would be parallel to and the remaining two lands would extend at right angles to the longitudinal axis of the sleeve. It is suggested that such orientation of the lands is more likely to prevent undesirable tilting in a plane which is normal to the longitudinal axis of the sleeve.
An object which is common to the inventions disclosed in the aforediscussed patents is to ensure that the marginal portion of the exposed external surface of each bearing shoe be maintained at a constant distance from the internal surface of the sleeve, i.e., that each and every portion of such marginal portion be disposed at a predetermined distance from the sleeve. This is intended to guarantee the formation of a liquid film having a constant thickness and to prevent direct surface-to-surface (metal-to-metal) contact between the bearing shoes and the sleeve. It has been found that the proposals of Biondetti and Spillmann et al. cannot ensure total absence of metal-to-metal contact between the bearing shoes and the sleeve under any and all circumstances which arise when a roll of the above outlined charcter is in actual use. In many instances, the shoes are likely to destroy the liquid films between their external surfaces and the internal surface of the sleeve, especially when the roll is idle (i.e., when the sleeve does not rotate about the shaft) and/or during the initial stage of acceleration of the sleeve from zero speed.